make es bulk request response time retrievable

Hi Team,

I have asked a question in fluent-plugin-elasticsearch and they'd like me to send a request here.
For the issue backgroud, we are using the fluentd to send logs to elasticsearch now and we found that there are a lot of "slow_flush" warning in the fluentd log. I understand that the fluentd send logs to elasticsearch by using the bulk api request. Therefore, we checked if there is any slow_index in elasticsearch performance but it seems the elasticsearch performance is good and the response time and not very large.
Therefore, I would like to know is there any parameter that I can add to fluentd config to enable checking the response time of the bulk request?